The present invention relates to apparatus for controlling the level of a liquid in a container.
In particular, the present invention relates to apparatus for controlling the level of lubricating oil in the oil pot of a motor, the invention having particular application to reactor coolant pump motors in nuclear power plants.
The reactor coolant pump motors drive the reactor coolant pumps which are part of the reactor coolant system in a nuclear power plant. The motors are typically arranged with vertical shafts. The bearing and lubrication system of a vertical motor is usually contained in two separate oil pots. An upper oil pot contains the upper guide or radial bearings and the total thrust bearing system. A lower oil pot contains the lower guide or radial bearings. Each of these oil pots is typically provided with cooling coils for carrying cooling water to dissipate the heat which is generated by the bearing systems.
The design of the oil pots is such that the oil level within the pot should be monitored during operation to ensure that the oil level is not rising above or falling below expected levels. A rising level might indicate, for example, a water leak within the cooling coils which results in water entering the pot and mixing with the oil. If such a situation were to persist, the lubricating ability of the oil would be sharply diminished and, more importantly, the oil/water mixture would overflow the pot and migrate toward the hot reactor coolant pump, where a fire would almost certainly result.
A falling level would be indicative of a leak in the oil pot system which allows oil to escape from the pot. If this situation persists, the level of the oil in the pot will drop below the level where the oil lubricates the bearings and thus result in severe damage to the bearings and possibly to the motor shaft/runner. More importantly, this condition could also result in a fire if the oil, with a flash point of 420.degree. F. reaches the pump surfaces which may be as hot as 550.degree. F.
Because of these very real and serious concerns, each of the two oil pots is equipped with an oil level detector which provides an alarm signal to a control room in the event of an unusual oil level condition. Some concern exists that the detector may generate a high level alarm when, in fact, the system is operating normally, i.e., there is no leakage of water into the oil pump. A major contributor to this potential problem, particularly with respect to the lower oil pot, is the expansion of the oil due to heat entering the oil pot from the reactor coolant pump. A temperature rise of 50.degree. F. in the oil of the pot, for example, would result in a volume expansion of approximately 0.6 gallon in a 30-gallon capacity pot. This is reflected in the rise of the oil level within the pot and the detector of 0.5 in or more, and could result in a spurious high level alarm signal.